Most audible devices rely upon some form of loudspeaker transducer to transform electrical signals into acoustic waves. These transducers are anything but perfect devices, and introduce numerous forms of distortion into the transformation process. One particularly troublesome characteristic of most loudspeakers is the fact that the impedance is non-linear with respect to both frequency and excitation level. A small variation in the loudspeaker can yield a major variation in perceived performance.
Prior systems utilize either voltage or current control to address the variable impedance presented to a driver by a loudspeaker. However, controlled acoustic power remains an elusive goal. Generally, a loudspeaker transducer's impedance increases as the frequency applied to the transducer decreases. Accordingly, a voltage-controlled amplifier driving a loudspeaker transducer is limited by the increasing impedance in that, below a certain frequency, the current put through the increased impedance is too low to produce acceptable levels of sound. A current-controlled amplifier is able to produce sound at these lower frequency, higher transducer impedance points, but suffers from a risk of ruining the loudspeaker. As the impedance increases and the amplifier continues to put out constant current, the voltage can rise unacceptably high, blowing out the speaker.
Accordingly, an improved method for controlling a signal applied to a loudspeaker transducer is needed.